In a liquid crystal display device, liquid crystal is a substance that does not itself illuminate light. Instead, the liquid crystal relies on reflecting light from a light source, thereby displaying images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
Generally, backlight modules can be classified into an edge lighting type or a bottom lighting type based upon the location of lamps within the device. The edge lighting type backlight module has a lamp unit arranged at a side portion of a light guiding plate for guiding light. The edge lighting type backlight modules are commonly employed in small-sized LCD due to their lightweight, small size, and low electric consumption. However, the edge lighting type backlight modules are not suitable for large-sized LCD (20 inches or more). A bottom lighting type backlight module has a plurality of lamps arranged at regular positions to directly illuminate an entire surface of an LCD panel. The bottom lighting type backlight modules have a higher efficiency of light usage and longer operational lifetime than the edge lighting type backlight modules, the bottom lighting type backlight modules are especially used in large-sized LCD devices.
Referring to FIG. 6, a typical direct type backlight module 10 is shown. The backlight module 10 includes a light diffusion plate 11, a number of spacers 12, a plurality of lamps 13, and a housing 14. The housing 14 is substantially in the form of a rectangular block. A cavity 142 is defined in a center portion of the housing 14. The cavity 142 is used to accommodate the lamps 13 and the spacers 12 therein. The lamps 13 are arranged on a bottom surface inside of the housing 14. The light diffusion plate 11 is disposed on the housing 14 covering the cavity 142. The spacers 12 are positioned between the bottom surface of the housing 14 and the light diffusion plate 11, so as to support the light diffusion plate 11. The backlight module 10 further includes a reflective film 15 deposited on an inner surface of the housing 14, for improving the backlight module 10 light energy utilization rate.
In use, light rays emitted by the lamps 13 are directly emitted into the light diffusion plate 11, and a substantial remainder of the light rays emitted from the lamps 13 that does not directly emit into the light diffusion plate 11 are reflected to the light diffusion plate 13 by the reflective film 15. Some of the light rays are reflected by an incident surface of the light diffusion plate 11, and these are reflected back to the light diffusion plate 11 by the reflective film 15. The light rays are diffused in the light diffusion plate 11, and thus surface light rays are output from an LCD panel (not shown) stacked on the light diffusion plate 11.
However, to enhance uniformity of light rays for the backlight module 10, there must be a certain space between the light diffusion plate 11 and the lamps 13 for eliminating potential dark strips caused by the reduced intensity of light between adjacent lamps 13. Therefore, it suffers from increased thickness and decreased luminance due to the space exiting between the light diffusion plate 11 and lamps 13. In addition, the light diffusion plate 11 is typically manufactured by uniformly dispersing a plurality of light diffusion particles into transparent resin matrix materials. Because the light rays are diffused at the light diffusion particles many times in the light diffusion plate 11, a part of the light energy would have been consumed in the light rays' diffusing process, thus a light brightness of the backlight module is decreased.
What is needed, therefore, is a direct type backlight module that overcome the above mentioned disadvantage.